Evidence of limited carbon sequestration in soils under no-tillage systems in the Cerrado of Brazil

Abstract

The Brazilian government aims at augmenting the area cropped under no-tillage (NT) from 32 to 40 million ha by 2020 as a means to mitigate CO 2 emissions. We estimated soil carbon (C) sequestration under continuous NT systems in two municipalities in the Goiás state that are representative of the Cerrado. A chronosequence of NT fields of different age since conversion from conventional tillage (CT) was sampled in 2003 and 2011. Soil C levels of native Cerrado and pasture were measured for comparison. After about 11 to 14 years, soil C stocks under NT were highest and at the levels of those under natural Cerrado. Average annual rates of soil C sequestration estimated using the chronosequence approach were respectively 1.61 and 1.48 Mg C ha −1 yr −1 for the 2003 and 2011 sampling, and were higher than those observed using repeated sampling after eight years. The diachronic sampling revealed that the younger NT fields tended to show higher increases in soil C stocks than the older fields. Converting an extra 8 million ha of cropland from CT to NT represents an estimated soil C storage of about 8 Tg C yr −1 during 10 to 15 years.At the Copenhagen Climate Change Conference of 2009 the Brazilian government committed to mitigation actions leading to projected reductions in its greenhouse gas emissions of 36 to 39% by 2020 1 . Brazil is one of the major greenhouse gas emitters in the world with historically more than half of its emissions originating from deforestation for agricultural land. From 2005 to 2010 reduced rates of deforestation resulted in circa 40% decrease in the total Brazilian emissions 2 . Today, emissions from agriculture, originating from cattle ranching and to a lesser extent from chemical fertilizer use, are at least equivalent to those from deforestation. Beside the continued efforts to control and reduce emissions from deforestation, one of the mitigation measures is the implementation of strategies that maintain and increase stocks of organic carbon (C) in agricultural soils. The adoption of no-tillage (NT) systems in croplands is considered as an effective way to achieve this. Within its ' Action Plan for Low Carbon Agriculture' launched in 2010, the Brazilian government aims at augmenting the area cropped under NT from 32 to 40 million ha by 2020 3 . Several studies suggest that a change from conventional tillage (CT) to NT cropping systems leads to an increase in soil C, especially if this occurs with intensification of crop production 4-7 .In Brazil, the Cerrado region is regarded as the target region for the efforts in mitigating CO 2 emissions through the adoption of NT systems 8 . The Cerrado occupies about 23% of the Brazilian territory or about two million km 2 in the central part of the country 9 . The region has experienced a rapid expansion of large-scale commercial agriculture since the early 1970s, when governmental policies were put in place designed to increase production of commodities for export as a response to the increased global demand for soybean and meat 10,11 . From a tot...

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